1. Field of the Invention
The present invention relates generally to surface preparation. More particularly, the invention concerns an improved surface preparation apparatus for forming a very fine surface on wooden workpieces such as cabinet doors and the like.
2. Discussion of the Prior Art
A number of different kinds of handheld as well as volume production type machines for preparing surfaces of various types of workpieces have been suggested in the past. These machines typically use a sanding belt entrained around a sanding platen. The major drawback of many of these machines is that they often leave unsightly cross-grain scratch patterns in the surface of the workpiece. Frequently hand sanders such as those disclosed in U.S. Pat. No. 4,487,010 issued to Dicke are used to remove the unacceptable cross-scratches. However, this process is very labor intensive, time consuming and expensive. Further, hand sanding generally fails to produce a uniform surface particularly on relatively large surfaces, such as cabinet doors.
In an attempt to overcome the aforementioned drawbacks of prior art sanding processes, various designs of production type apparatus having one or more oscillating sanding heads have been proposed. One such apparatus is described in U.S. Pat. No. 5,081,794 issued to Haney. The Haney patient describes a dual orbting sanding apparatus that includes a frame a conveyor, first and second stepped drive shafts that support a brace and cause the brace to move in a first orbit. The apparatus further includes second and third stepped drive shafts that are supported by the brace and are connected to the platen to move the platen in a second orbit.
U.S. Pat. No. 2,787,100 issued to Peyches discloses a machine for grinding or polishing glass. More particularly, the patent describes a polisher wherein a slurry or suspension containing the abrasive grit is continuously fed into the machine as the work travels through the machine. While the machine produces a circular motion combined with a slow reciprocating motion, these motions are induced by totally different types of mechanisms from those of the apparatus of the present invention.
A German Patent No. 27 40 696 issued to Meyer concerns a grinder or polisher for grinding tombstones. The Meyer apparatus includes a bridge on which a grinding head is mounted. The grinding head powers a rotating disk grinder. In operation, the grinding head along with the disk grinder must traverse the entire face of a tombstone in order for polishing operation to be accomplished.
In most of the prior art orbital sanders, the orbital, or oscillatory movement of the platen is accomplished using some type of stepped shaft or crank mechanism. Generally speaking in such devices, the higher the rate of rotation of the drive shafts, the better will be the performance of the sander. However, as the speeds of rotation increase bearing wear, including wear on bearings attached to the platen, can become excessive resulting in frequent bearing failure to mitigate against excessive platen bearing wear, the platen size of the sander must, of necessity, be kept small thereby limiting the effectiveness of the machine for use in high volume production processes. Stated another way, as the orbiting platen becomes larger and heavier, the size of the off-set bearing must be increased to withstand the tremendous forces created on the bearing as the platen orbits.
As will be better understood from the description that follows, the apparatus of the present invention uniquely overcomes the mechanical limitations inherent in prior art devices which embody crank or stepped shaft type drive mechanisms to obtain orbital movement of the platen by providing a highly novel orbit generating mechanism to produce a controlled orbital movement to the platen. Additionally, the apparatus of the invention includes a unique vacuum sanding platen and novel means for removably attaching the sandpaper to the platen.